If you want to go to Nepal there are direct buses to kathmandu. but it will take more than 30 days to reach there by bus. You can board a flight from Delhi or Dehradun to Kathmandu. It will be costly but it will be more comfortable journey. There are no direct trains. You can go to Sikkim by train and from there you can board a bus to Kathmandu. Take a flight journey if you have good budget. If you have lots of time then choose either of train or bus. Happy Journey
Its better. If you can go to gorakhpur. From there you can go to sonauli border. I just came from there. Its so easy just took 2 mins to cross the border. once you reach to Nepal buses are available to pokhara and kathmandu.